1. Field of the Invention
The present invention relates to thin-film multilayered electrodes, and also relates to high-frequency transmission lines, high-frequency resonators, high-frequency filters, and the like which include the thin-film multilayered electrodes.
2. Description of the Related Art
With the recent advances in miniaturized electronic components, miniaturization of devices has been attempted also by using materials which have a high dielectric constant in high frequency bands such as microwaves, submillimeter waves, or millimeter waves. If dimensions are reduced by using materials having a high dielectric constant, however, energy loss increases inversely with the cube root of volume. Energy loss in high-frequency devices can be broadly classified into conduction loss caused by skin effects and dielectric loss caused by the dielectric materials. Recently dielectric materials having a high dielectric constant and at the same time low loss characteristics have been put to practical use, and thus conduction loss dominates energy loss in comparison with dielectric loss.
Under these circumstances, in PCT Patent Publication No. WO95/06336, the assignee of the present invention disclosed a thin-film multilayered electrode which is capable of decreasing conduction loss in the high frequency band, and also disclosed a method for designing an optimum thickness for each layer in the thin-film multilayered electrode at a specific operating frequency. FIG. 4 is a perspective view of a half-wavelength transmission line-type resonator 101 which includes a thin-film multilayered electrode 103 formed in accordance with the design method disclosed in PCT Patent Publication No. WO95/06336.
As shown in FIG. 4, the half-wavelength transmission line-type resonator 101 includes a dielectric substrate 102 provided with a ground conductor 106 on the entire back surface, and the thin-film multilayered strip electrode 103, placed on the dielectric substrate 102, having a length of .lambda.g/2 (.lambda.g is a guide wavelength) in the longitudinal direction.
As illustrated in FIG. 5, in the thin-film multilayered electrode 103, a thin-film conductive layer 104a is formed on the surface of the dielectric substrate 102, and a thin-film dielectric layer 105a is deposited on the thin-film conductive layer 104a. Thenceforth, thin-film conductive layers 104b, 104c, and 104d and thin-film dielectric layers 105b and 105c are alternately stacked in that order to form the thin-film multilayered electrode 103. A length in the longitudinal direction of the thin-film multilayered electrode 103 is set at a half-wavelength of a desired frequency, enabling it to function as a resonator.
At this stage, a TEM mode microstrip line (hereinafter referred to as a principal transmission line) 107 is formed by the thin-film conductive layer 104a, the ground conductor 106 (FIG. 4), and the dielectric substrate 102. Also, on the principal transmission line 107, a TEM mode sub-transmission line is formed by the thin-film dielectric layer 105a interposed between a pair of thin-film conductive layers 104a and 104b. The thin-film dielectric layers 105b and 105c similarly form sub-transmission lines. With respect to the conventional thin-film multilayered electrode 103, by using the method disclosed in PCT Patent Publication No. WO95/06336,
(a) a thickness and a dielectric constant .di-elect cons. of each of the thin-film dielectric layers 105a, 105b, and 105c is set so that phase velocities of TEM waves which propagate through the principal transmission line 107 and the individual sub-transmission lines are substantially identical with each other; and PA1 (b) a thickness of each of the thin-film conductive layers 104a, 104b, and 104c is set at a predetermined thickness which is smaller than a skin depth of an operating frequency so that electromagnetic fields between the principal transmission line 107 and its adjacent sub-transmission lines and between the individual sub-transmission lines are coupled with each other.
Accordingly, a portion of high-frequency energy which flows through the principal transmission line 107 is transferred to the sub-transmission lines, and the high-frequency electric current flows through each of the thin-film conductive layers 104a, 104b, 104c, and 104d, and thus the skin effect of the electrode in the high-frequency region can be substantially suppressed.
In accordance with the thin-film multilayered electrode disclosed in PCT Patent Publication No. WO95/06336, the thickness of each thin-film conductive layer and thin-film dielectric layer is set on the precondition that the thin-film multilayered electrode be formed on a dielectric substrate 102 having a flat surface (for example, a mirror-polished sapphire substrate composed of single-crystal alumina).
In the case of using, for example, a ceramic substrate as a dielectric substrate, however, the surface of the substrate is uneven or rough because of the existence of pores or the like. Although the unevenness can be planarized up to a point by, for example, surface polishing treatment, the surface of the substrate cannot be polished sufficiently because there are many pores in the substrate as well as on the surface, and new pores may be exposed during the polishing treatment. FIG. 6 is a sectional view of a layered structure in which a thin-film multilayered electrode is formed on an uneven dielectric substrate. As shown in FIG. 6, each of the thin-film conducting layers and thin-film dielectric layers will be uneven in accordance with the unevenness of the substrate. If each layer is formed unevenly in such a manner, phase velocities of TEM waves which propagate through the principal line and the individual sub-transmission lines cannot be equalized as originally designed. Also, when thin-films are deposited on an uneven substrate, two adjacent thin-film conductive layers may easily be short-circuited during the deposition process. Such conditions interfere considerably with the effective suppression of the skin effect by the thin-film multilayered electrode.